The prior art discloses methods in which a plurality of switches that are connected in series have to be closed in order to connect a photovoltaic installation to a medium-voltage grid. This results, for example, from different regulations or responsibilities relating to switches on the low-voltage side in comparison with those on the medium-voltage side. Generally, the switches on the medium-voltage side cannot be operated by a controller of the photovoltaic installation; it is often a case of switchgear systems that can only be switched manually. The switchgear systems remain closed in normal operation and are only opened in the event of faults and for maintenance work. In many cases, maintenance personnel are not authorized to switch the medium-voltage switchgear system.
If there is at least one switch present both on the medium-voltage side and on the low-voltage side, the switch on the medium-voltage side can thus remain closed and the switch on the low-voltage side then serves the purpose of connecting the photovoltaic installation to the medium-voltage grid. The switch on the low-voltage side can be actuated by the controller of the photovoltaic installation; the switches are generally expensive, motor-driven switches because large currents flow through them. In this case, voltage detection means are usually provided on both sides of the switch because, even in the case of an open switch, for example, the voltage level of the grid voltage has to be identified in order to establish whether it is possible to connect to the grid.
In particular, connection is permitted only in the presence of the normatively predefined grid connection conditions and synchronization conditions (amplitude identity and in-phase nature of the grid and voltage to be fed).
Connection standards also specify the accuracy with which a check, for example of the grid voltage, has to be carried out. For reasons of safety, a further voltage detection means is usually provided on the medium-voltage side; here it is often a voltage detection means having a capacitive voltage divider. Such a voltage detection means having a capacitive voltage divider is usually subjected to lower requirements in terms of accuracy, but for reasons of safety should at least indicate whether, for example, 50% of the grid voltage is applied.
Disadvantages of this design are the multiplicity of voltage detection means, the Joule heat losses in the low-voltage switch and the costs of the switch on the low-voltage side.
DE 10 2013 202 868 A1 has already disclosed a voltage measuring arrangement having a reference voltage measuring apparatus that is designed with a higher degree of measurement accuracy than a capacitive voltage measuring apparatus. The measurement location of the reference voltage measuring apparatus is located in another field or on the busbar but at any rate is electrically connected to the measurement location at which the capacitive voltage measurement is performed.
DE 10 2012 105 721 A1 from the same applicant discloses a method for controlling a plurality of inverters that are connected on the input side in each case to a current source and on the output side to a common grid transfer point, in which electrical variables at the grid transfer point are measured and simultaneously correlated with the electrical variables measured at the individual inverters.
DD 2 05 257 A1 describes a high-voltage measuring device having a capacitive voltage divider, wherein the voltage values are subsequently amplified for analog data transmission by means of an amplifier circuit.
In order to dispense with the expensive, motor-driven switch on the low-voltage side, it is known to provide a motor-driven medium-voltage power switch and to use it to connect the photovoltaic installation to the medium-voltage grid. Since lower currents are required on the medium-voltage side on account of the higher voltage level given the same power, the motor-driven medium-voltage power switch offers advantages in terms of cost. In this case, the voltage detection means on the medium-voltage side has to satisfy higher demands in terms of accuracy. The grid connection standards have to be able to be checked with sufficient accuracy before the photovoltaic installation is connected, that is to say when the medium-voltage power switch is open. Voltage detection means that contain an isolating transformer are used for this purpose. The voltage detection means decrease the medium voltage to such an extent that it can be measured with a high degree of accuracy by means of established methods. However, such voltage detection means having isolating transformers are large and expensive on account of the high isolating voltage.